Theodore's one-timer from the right point hit off the end boards and went in off Brandon Bussi after Carolina forward Jordan Martinook touched the puck and had it go off the goalie.
Vegas leads the best-of-7 series 2-1. Game 4 is here Tuesday (8 p.m. ET; ABC, SN, TVAS, CBC).
Mitch Marner scored the fastest hat trick in Stanley Cup Final history with three consecutive goals in a span of 6:10 to give Vegas a 4-0 lead with 3:08 remaining in the second period. Marner bested Maurice Richard's 69-year-old record of three goals in 6:21.
Marner also had an assist on Tomas Hertl's power-play goal 16 seconds before he deposited his first as the Golden Knights built the four-goal lead in a 6:26 span. He is the first NHL player to record four points in a single period in Cup Final history.
Marner had a chance at a fourth goal with a short-handed penalty shot at 4:04 of the third period, but Bussi, who replaced Frederik Andersen to start the third period, stopped him, and 2:59 later the Hurricanes started their comeback.
Martinook, Taylor Hall and Jordan Staal scored 39 seconds apart on three straight shots, the three fastest goals in Stanley Cup Final history, to make it 4-3 with 12:18 remaining and break the 72-year-old record set by the Montreal Canadiens. Montreal scored three goals in 56 seconds in Game 2 of the 1954 Final.
Andrei Svechnikov then scored the tying power-play goal with Bussi pulled for an extra skater with 1:42 remaining in regulation.
The Hurricanes became the second team in Stanley Cup Final history to erase a four-goal deficit in a game. The New York Rangers did it in Game 1 in 1972 but lost to the Boston Bruins in regulation.
Carter Hart made 29 saves for the Golden Knights.
Andersen allowed four goals on 16 shots. Bussi made 18 saves on 19 shots.
Vegas had two goals disallowed in the second period, at 34 seconds (offside) and 4:00 (goaltender interference), because of successful coach's challenges by Carolina.
Hertl scored the first Vegas goal that counted at 10:26 of the second, connecting 10 seconds into the first power play of the game for either team to give the Golden Knights a 1-0 lead.
Vegas was 0-for-7 on the power play in Games 1 and 2.
Marner scored 16 seconds later, when his backhanded shot from near the wall in the right face-off circle was inadvertently knocked into the net by Carolina defenseman Sean Walker, giving Vegas a 2-0 lead.
Marner made it 3-0 at 14:32, 11 seconds after he hit the right post on a breakaway. Brayden McNabb kept the puck in the zone and found Marner low behind all five Carolina skaters, where he scored off a forehand-to-backhand deke for his second goal in 3:50 and Vegas' third in 4:06.
Marner's third goal came off another breakaway. He got loose past Alexander Nikishin, took a pass from Hertl, skated into the right circle and made it 4-0 with a far-side slap shot that went in off Andersen's blocker at 16:52.
Marner had a short-handed penalty shot at 4:04 of the third period, Bussi stopped him to keep it a 4-0 game.
Carolina mounted its comeback soon after Bussi made that save.
Martinook made it 4-1 at 7:03, cashing in off a strong forecheck by Seth Jarvis before getting hit from behind by Cole Smith.
Hall cut the deficit to 4-2 at 7:29. Sebastian Aho stole the puck from McNabb in the offensive zone and from below the right circle sent a backhanded pass across to Hall, who scored from the far post.
Staal made it 4-3 at 7:42, scoring from the lower part of the left circle on a deflection of Jaccob Slavin's shot off the left-wing half-wall.
Carolina also scored three goals on consecutive shots in the third period of Game 2 on Thursday.
Svechnikov tied it 4-4 at 18:18, scoring a power-play goal with Bussi pulled for the extra skater to make it a 6-on-4 situation. After the puck went to the net and was loose, Carolina forward Nikolaj Ehlers was pushed in as Svechnikov found it in the slot and shoved it into the net.
